One of means for deciding the exposure of a digital camera is a scan type exposure decision apparatus for performing photometry at predetermined exposure values and deciding optimal exposure on the basis of the photometry data.
According to this method, photometry is done at a plurality of exposure values, and the exposure difference value between the brightness value of a screen and a target brightness value at each exposure value is calculated. The exposure value of actual exposure is decided by using an exposure value having the smallest absolute value of the exposure difference value out of images obtained at these exposure values, and by using the exposure difference value of this exposure value.
However, this apparatus may not accurately calculate an exposure difference value under the influence of the saturation characteristic of a CCD or the like.
The method of obtaining an exposure value in the apparatus will be exemplified.
In FIG. 5, the ordinate represents the level of the exposure value.
Letting BvT0 be the exposure value of an object, exposure difference values upon exposure at exposure values Bv0 and Bv1 are DeltaBv0 and DeltaBv1. The exposure value difference upon exposure at the exposure value Bv0 does not become DeltaBv0′ because the value is clipped due to the saturation characteristic of the CCD and Bv0 is output as a value smaller an actual value.
Comparing the absolute values of DeltaBv0 and DeltaBv1, DeltaBv1 is smaller. An exposure value BvT0 is therefore attained by Bv1+DeltaBv1.
If accurate values of both Bv0 and Bv1 cannot be obtained, as shown in FIG. 6, neither Bv0+DeltaBv0 nor Bv1+DeltaBv1 may coincide with BvT1.
This phenomenon prevents accurate determination of the exposure value of an object.
To attain accurate exposure by this method, the number of exposure scan operations must be increased in consideration of CCD saturation and the presence of a nonlinear region. Exposure must be more finely set to minimize errors under the influence of saturation or the like.
This scan must be performed when the user presses the shutter. An increase in the number of scan operations directly leads to an increased shutter time lag.